1. Field of the Invention
The present invention relates generally to a suspension control apparatus for a vehicle which can suppress rolling, nose diving, squatting and vertical motions such as pitching and bouncing of a vehicle so as to provide a more comfortable ride. More particularly, it relates to a suspension control apparatus which serves to inhibit normal vehicle-height control during active suspension control operations but start such height control immediately after the active suspension control operations have been finished. 2. Description of the Related Art
In the past, many suspension control apparatus of this type have been known, one example of which is disclosed in Japanese Utility Model Laid-Open No. 61-163710. In this conventional suspension control apparatus, a fluid spring chamber such as a pneumatic spring chamber is interposed between each road wheel and the body of a vehicle so that supply to and discharge from the fluid spring chamber of compressed air is controlled in an appropriate manner to suppress rolling or nose diving of the vehicle body. Specifically, there is a tendency that when a vehicle is steered to turn, the suspension units disposed on the turning side of the vehicle are forced to contract under the action of centrifugal force whereas those on the opposite side thereof expand. In order to suppress this tendency, a predetermined amount of compressed air is supplied to the fluid spring chamber of each of the suspension units on the contracted side, and at the same time a predetermined amount of pressure air is discharged from the fluid spring chamber of each of the suspension units on the expanded side so as to incline the vehicle body in the opposite direction, thereby maintaining the attitude of the vehicle body in a horizontal manner.
With the conventional suspension control apparatus as constructed above, however, for the purpose of effectively performing control of the vehicle attitude such as roll control, nose dive control, squat control and the like, it was first proposed that a change in the height of a vehicle is detected by a vehicle-height sensor so that the vehicle height is adjusted based on the results thus obtained. But such a vehicle-height adjusting technique was subject to time lags in operation and can not attain high control performance.
To improve this, the vehicle-attitude control such as roll control and the like is carried out using quantities for attitude control which are determined in an open loop manner. In this case, however, because of the open loop control, there is the possibility that the average vehicle height during roll control does not match a target vehicle height in the case of an extreme rolling condition such as great rolling.
To correct or accommodate such a deviation or difference between the average and the target vehicle height by employing normal vehicle-height control can not meet the rolling condition of the vehicle or can not effectively suppress rolling of the vehicle due to too large time lag n control operation.
Further, when roll control is finished and the vehicle is restored to normal running attitude, the height of the vehicle does not often return to the average height which it took before commencement of the roll control. In the case of returning the vehicle height to the average level by means of normal height control, it is usual practice not to make vehicle-height control follow a rapid change in the vehicle height in a short time which requires control of the vehicle attitude. As a result, there is the drawback that it takes a relatively long time, e.g. 30 seconds, to correct or remedy the deviation of the vehicle height from the normal height level after finishing of the roll control.
Moreover, the above-described conventional suspension control apparatus, which performs active suspension control operations each in a open loop mode, also has another drawback in that a plurality of suspension control operations can not be conducted simultaneously with each other.